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| United States Patent |
5,261,967
|
|
Winston
, et al.
|
November 16, 1993
|
Powdered electric circuit assembly cleaner
Abstract
A method of removing rosin soldering flux or other residues from a printed
wiring board comprises directly adding to the wash water which contacts
the boards a powdered cleaning composition which comprises water soluble
alkaline salts. The method of directly adding the cleaning composition to
the water supply in the form of a powder eliminates the need to
incorporate organic stabilizers which previously have been required to
maintain surfactants and the like in solution in aqueous concentrates. By
reducing the organic content of the cleaning compositions, the aqueous
effluents from the cleaning method have substantially reduced biological
oxygen demand (BOD) and chemical oxygen demand (COD) levels.
| Inventors:
|
Winston; Anthony E. (East Brunswick, NJ);
Jones; Keith A. (Yardley, PA);
Cala; Francis R. (Highland Park, NJ);
Vinci; Alfredo (Dayton, NJ);
Lajoie; M. Stephen (Basking Ridge, NJ)
|
| Assignee:
|
Church & Dwight Co, Inc. (Princeton, NJ)
|
| Appl. No.:
|
896409 |
| Filed:
|
June 10, 1992 |
| Current U.S. Class: |
134/42; 134/2; 134/40; 510/175; 510/452; 510/453; 510/510; 510/511; 510/512 |
| Intern'l Class: |
B08B 003/04 |
| Field of Search: |
134/2,40,42
|
References Cited
U.S. Patent Documents
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|
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|
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| 3672993 | Jun., 1972 | Mitchell et al. | 134/3.
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| 3847663 | Nov., 1974 | Shumaker | 134/2.
|
| 3904685 | Sep., 1975 | Shahidi et al. | 260/537.
|
| 4089703 | May., 1978 | White | 134/22.
|
| 4214915 | Jul., 1980 | Dillarstone et al. | 134/19.
|
| 4228048 | Oct., 1980 | Tesdahl | 260/17.
|
| 4283299 | Aug., 1981 | Becker et al. | 252/90.
|
| 4304680 | Dec., 1981 | Wixon | 252/114.
|
| 4347152 | Aug., 1982 | Wixon | 252/174.
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|
| 4362640 | Dec., 1982 | Schreiber | 252/135.
|
| 4382825 | May., 1983 | McCready | 134/2.
|
| 4434069 | Feb., 1984 | Fairchild | 252/174.
|
| 4457322 | Jul., 1984 | Rubin et al. | 134/2.
|
| 4521332 | Jun., 1985 | Milora | 252/527.
|
| 4528039 | Jul., 1985 | Rubin et al. | 134/2.
|
| 4539133 | Sep., 1985 | Boskamp | 252/109.
|
| 4554026 | Nov., 1985 | Cosper et al. | 134/38.
|
| 4741863 | May., 1988 | Yamamoto et al. | 252/547.
|
| 4756846 | Jul., 1988 | Matsuura et al. | 252/156.
|
| 4783281 | Nov., 1988 | Bishop et al. | 252/135.
|
| 4820440 | Apr., 1989 | Hemm et al. | 252/135.
|
| 4820441 | Apr., 1989 | Evans et al. | 252/174.
|
| 4844744 | Jul., 1989 | Leiter et al. | 134/40.
|
| 4869844 | Sep., 1989 | Johnson | 252/159.
|
| 4904571 | Feb., 1990 | Miyashita et al. | 430/331.
|
| 5015408 | May., 1991 | Reuss | 252/99.
|
| 5049200 | Sep., 1991 | Brunner et al. | 134/2.
|
| 5096609 | May., 1992 | Dany et al. | 252/135.
|
| 5108641 | Apr., 1992 | Ahmed et al. | 252/94.
|
Primary Examiner: Garvin; Patrick P.
Attorney, Agent or Firm: Barris; Charles B.
Parent Case Text
This application is a continuation-in-part of U.S. Ser. No. 07/731,512
filed Jul. 17, 1991 and now abandoned.
Claims
What is claimed is:
1. A method of removing soldering flux alone or with other residues from a
printed wiring board, comprising
(a) providing a supply of water to a cleaning apparatus wherein said water
supply is contacted with said wiring board;
(b) directly adding to said water supply a cleaning composition in powdered
form comprising water soluble alkaline salts to form a cleaning solution;
(c) contacting the board with said aqueous cleaning solution; and
(d) removing the combined composition and soldering flux or other residues
from the board.
2. The method of claim 1 wherein said aqueous cleaning solution has a pH of
from about 10 to 13.
3. The method of claim 2 wherein said aqueous cleaning solution has a pH of
from about 10 to less than 12.
4. The method of claim 1 wherein said aqueous cleaning solution contains
from about 0.1 to 15 wt. % of said cleaning composition.
5. The method of claim 1 wherein at least one adjuvant selected from an
anticorrosion agent, antifoaming agent, surfactant and mixtures thereof is
added to the aqueous cleaning solution.
6. The method of claim 5 wherein said adjuvant is contained in said
powdered composition.
7. The method of claim 6 wherein said adjuvant is coated onto the powdered
alkaline salts.
8. The method of claim 7 wherein said adjuvant is liquid and sprayed onto
the powdered alkaline salts.
9. The method of claim 1 wherein said cleaning solution contains from about
0.6 to 15% by weight of the cleaning composition.
10. The method of claim 1 wherein said contact is carried out at a
temperature of from room temperature to about 180.degree. F.
11. The method of claim 10 wherein said contact is carried out for a period
of from 1 to 10 minutes.
12. The method of claim 6 wherein said adjuvant is in powdered form.
13. The method of claim 12 wherein said adjuvant is agglomerated with
powders of said alkaline salts.
14. The method of claim 1 wherein said cleaning composition is formed by
spray drying the components into powdered form.
15. The method of claim 5 wherein said adjuvant includes an anticorrosion
agent.
16. The method of claim 15 wherein said anticorrosion agent is an alkali
metal silicate characterized by an Alk.sub.2 O to SiO.sub.2 mole ratio of
between 1:0.5 to 1:4.5, wherein Alk represents an alkali metal.
17. The method of claim 16 wherein said alkali metal silicate is sodium or
potassium silicate.
18. The method of claim 15 wherein said anticorrosion agent is potassium
silicate.
19. The method of claim 1 wherein said cleaning solution is contacted with
the boards in the form of a spray.
20. The method of claim 1 wherein said cleaning solution is contacted with
said boards in the form of a bath in which said boards are immersed.
21. The method of claim 1 wherein said aqueous cleaning solution contains
from about 0.1-15 wt. % of said cleaning composition, said cleaning
solution having a pH of from about 10 to 13 and wherein said alkaline
salts are so combined so as to provide said cleaning solution with a
reserve of titratable alkalinity at least equivalent to from about 0.2 to
4.5% caustic potash when titrated to the colorless phenolphthalein
endpoint of about pH 8.4.
22. The method of claim 21 wherein at least one adjuvant selected from an
anticorrosion agent, antifoaming agent, surfactant and mixtures thereof is
added to the aqueous cleaning solution.
23. The method of claim 22 wherein said adjuvant is contained in said
powdered composition.
24. The method of claim 23 wherein said adjuvant is coated onto the
powdered alkaline salts.
25. The method of claim 24 wherein said adjuvant is liquid and sprayed onto
the powdered alkaline salts.
26. The method of claim 22 wherein said adjuvant is in powdered form.
27. The method of claim 25 wherein said adjuvant is agglomerated with
powders of said alkaline salts.
28. The method of claim 22 wherein said adjuvant includes an alkali metal
silicate anticorrosion agent.
29. The method of claim 28 wherein said anticorrosion agent is potassium
silicate.
Description
FIELD OF THE INVENTION
The present invention relates to environmentally safe powdered flux
removing compositions which are added directly to and dissolved in aqueous
media for cleaning electronic circuit assemblies, such as printed circuit
or printed wiring boards, during the fabrication thereof. Alkaline salts
are utilized, optionally with a corrosion inhibitor, antifoam agents and
other optional adjuvants such as surfactants to achieve a variety of
objectives, among which are the removal of solder flux, oils, waxes,
greasy substances, adhesive and other residues. A particular advantage of
the powdered product is the extremely low BODs and CODs imparted to the
water effluent.
BACKGROUND OF THE INVENTION
The cleanliness of electronic circuit assemblies (ECA), such as printed
circuit boards (PCB) or printed wiring boards (PWB), is generally regarded
as being critical to their functional reliability. Ionic and nonionic
contamination on circuit assemblies is believed to contribute to premature
failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic
contamination can accumulate after one or more steps of the process.
Circuit assembly materials are plated, etched, handled by operators in
assembly, coated with corrosive or potentially corrosive fluxes and
finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit
boards, soldering fluxes are first applied to the substrate board material
to ensure firm, uniform bonding of the solder. These soldering fluxes fall
into two broad categories: rosin and non-rosin, or water soluble, fluxes.
The rosin fluxes, which are generally only moderately corrosive and have a
much longer history of use, are still widely used throughout the
electronics industry. The water soluble fluxes, which are a more recent
development, are being used increasingly in consumer products
applications. However, the water soluble fluxes contain strong acids
and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately,
residues of any flux can cause circuit failure if residual traces of the
material are not carefully removed from the boards following soldering.
While water soluble fluxes can be easily removed with warm, soapy water,
the removal of rosin flux from printed circuit boards is more difficult
and has therefore traditionally been carried out with the use of
chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane,
trichloroethylene, trichloromonofluoromethane, methylene chloride,
trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or
mixtures or azeotropes of these and/or other solvents. These solvents are
undesirable, however, because they are toxic and when released into the
environment deplete the ozone layer and/or contribute to the greenhouse
global warming effect. Thus, use of such solvents is subject to close
scrutiny by the Occupational Safety and Health Administration (OSHA) and
the Environmental Protection Agency (EPA) and stringent containment
equipment must be used. Moreover, if released into the environment these
solvents are not readily biodegradable and are thus hazardous for long
periods of time.
In view of the toxicity of such solvents and, in view of recent concerns
regarding the release of chlorofluorocarbons into the environment, the use
of such solvents is being heavily regulated and phased out such that
alternatives to such solvents are in immediate need.
Alkaline cleaning compounds known as the alkanolamines, usually in the form
of monoethanolamine, have been used for rosin flux removal as an
alternative to the toxic chlorinated hydrocarbon solvents. These high pH
compounds (e.g., about 12 pH), chemically react with rosin flux to form a
rosin soap through the process of saponification. Other organic substances
such as surfactants or alcohol derivatives may be added to these alkaline
cleaning compounds to facilitate the removal of such rosin soap.
Unfortunately, these high pH compounds, like the water soluble soldering
fluxes, have a tendency to cause corrosion on the surfaces and interfaces
of printed wiring boards if they are not completely and rapidly removed
during the fabrication process. Additionally, these cleaning compounds
provide high levels of organics to the wash bath. Thus, the water
effluents obtained during the cleaning process must be treated to bring
the COD and BOD to acceptable levels before disposal.
In other approaches, Daley et al., U.S. Pat. No. 4,635,666 utilize a highly
caustic solution having a pH of 13 in a batch cleaning process. This
method severely oxidizes the solder applied to the circuit board. In Hayes
et al., U.S. Pat. Nos. 4,640,719 and 4,740,247 rosin soldering flux and
other residues are removed from electronic assemblies by means of terpene
compounds in combination with terpene emulsifying surfactants by rinsing
in water.
The complete removal of adhesive and other residues also poses a problem.
During the manufacture of electronic circuit assemblies the components are
mounted on the upper surface of the board with leads protruding downwardly
through holes in the board and are secured to the bottom surface of the
board by means of an adhesive. Further, it is sometimes necessary to
temporarily protect certain portions of the board from processing steps
such as the process of creating corrosion resistant gold connecting tabs
at the board edges. This transient protection of portions of the circuit
board can be achieved by the application of special adhesive tape to
susceptible areas. Once such protection is no longer needed, the adhesive
tape must be removed. In both instances, a residue of adhesive generally
remains which, if not thoroughly removed, can cause premature board
failure. Removal of this adhesive residue has traditionally been carried
out by the use of chlorinated solvents which, as already described, are
toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic
circuit assemblies and which can be removed by the compositions and method
of the present invention include, but are not limited to, for example,
rosin flux, photoresist, solder masks, adhesives, machine oils, greases,
silicones, lanolin, mold release, polyglycols and plasticizers.
In copending, commonly assigned U.S. Ser. No. 731,512, filed Jul. 17, 1991,
an improved cleaning composition characterized by non-corrosiveness and
low environmental impact, unlike the prior art chlorinated hydrocarbon
solvents and alkaline cleaners, are employed for cleaning printed wire
board and printed circuit boards. As disclosed therein, the cleaning
compositions comprise alkali metal carbonate and bicarbonate salts so
combined that during use at the desired concentration of these salts, the
pH of the wash solution ranges from about 10 to 12, typically
substantially less than 12 and even less than pH 11. Such cleaners are
very effective in removing all the flux and other residues from the
circuit and wiring boards. Moreover, since the carbonate and bicarbonate
salts are nontoxic and are very compatible with the environment, the use
of the cleaning compositions disclosed in the above-mentioned application
represent a substantial improvement in the art.
The marketplace has become readily accustomed to and therefor have demanded
that the above-described cleaning composition and other cleaning
compositions be provided in the form of aqueous concentrates which are
added to the wash water during the cleaning process. Unfortunately, by
providing cleaning compositions in the form of aqueous concentrates,
organic stabilizers are required to maintain certain adjuvants such as
anticorrosion agents and surfactants in aqueous solution in the
concentrate during handling and storage and the like. These additional
organics in the aqueous concentrate add to the BOD and COD levels of the
water effluents from the washing process. It would be very advantageous to
remove this minimal although still present environmental burden from these
aqueous cleaning compositions used for cleaning the electronic circuit
assemblies as described above.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide compositions and
methods for the safe and effective removal of rosin soldering fluxes from
electronic circuit assemblies, e.g., printed circuit boards, without
otherwise adversely affecting the boards. It is a further objective of
this invention to provide safe and effective compositions and methods for
the removal of other residual contaminants from printed circuit
assemblies.
Another important objective of the present invention is to provide an
effective cleaning composition and method for the safe and effective
removal of rosin soldering fluxes from electronic circuit assemblies and
which impart the barest minimum levels of organics to the wash water.
This invention provides cleaning compositions and methods for the removal
of rosin solder fluxes and other residues during the fabrication of
printed circuit or wiring boards. As a result, the possibility of
premature circuit failure that might occur in the absence of such cleaning
is eliminated or greatly reduced. The cleaning efficacy of the
compositions of the invention is such that printed wiring boards thus
treated meet stringent U.S. Department of Defense specifications.
The compositions of the invention are characterized by non-corrosiveness
and very low environmental impact, unlike the chlorinated hydrocarbon
solvents and highly alkaline cleaners that have heretofore been employed
for printed wiring board and printed circuit board cleaning.
Advantageously, the flux removing compositions, as used herein, exhibit
very low biological oxygen demands (BOD) and chemical oxygen demands (COD)
and may even allow discharge of water effluents to the sewer without
further treatment.
The present invention provides powdered printed circuit/wiring board
cleaning compositions comprising water soluble alkaline salts so combined
that they yield, when used in concentrations of about 0.1 to 15 percent by
weight in water, a pH of from about 10 to 13, preferably, from 10 to less
than 12. It is desirable for effective cleaning that the composition be in
amounts at the dilution of the wash bath to provide an adequate reserve of
titratable alkalinity, at least equivalent to from about 0.2 to 4.5
percent caustic potash (potassium hydroxide), when titrated to the
colorless phenolphthalein end point, which is about pH 8.4. The cleaning
compositions of this invention are used by adding the powdered cleaner
directly into the wash bath.
The powdered flux removing compositions also preferably contain a corrosion
inhibitor to provide corrosion protection for the metal surfaces being
cleaned. The powdered flux removing compositions may also contain a small
amount of an antifoam agent or a surfactant with foam reducing properties,
wetting agents, surfactants, and emulsifiers. These adjuvants can be
included with the powdered salts per se such as by coating the powders or
these adjuvants may be added separately to the aqueous wash bath. Since
the cleaning composition is provided in powdered form, there is no need to
include hydrotropes or other organic stabilizers to maintain surfactants,
corrosion inhibitors, and other adjuvants in solution such as needed in
aqueous concentrate packages. Accordingly, the powdered compositions of
this invention yield reduced BOD and COD levels in the aqueous effluents
from the cleaning process.
Moreover, when used according to the above, the compositions do not leave
an undesirable residual film and are effective in removing the fluxes and
other residues from electronic circuit boards.
DETAILED DESCRIPTION OF THE INVENTION
The objects and advantages mentioned above as well as other objects and
advantages may be achieved by the compositions and methods hereinafter
described.
Essentially, the powdered flux removing compositions of the invention
comprise mixtures of alkali metal salts which form the bulk of the
cleaners. Adjuvants, such as, corrosion inhibitors, antifoam agents and
surfactants are preferably added to enhance performance of the cleaning
composition. Accordingly, the term "flux removing compositions" as used
herein is intended to define the mixture of active ingredients comprised
of the alkali metal salts and, preferably, corrosion inhibitors, antifoam
agents, surfactants and any other adjuvants as hereinlater described.
As used herein the terms "flux removing solutions" or "flux removing
solutions in use" is meant to define aqueous mixtures of from about 0.1
to 15 percent by weight of the flux removing composition with the balance
comprised essentially of water which are employed in the cleaning methods
of the invention.
In accordance with the invention, additives, adjuvants, or the like, may be
included with the flux removing compositions and form part of the powdered
compositions or the flux removing solutions in use.
The flux removing compositions of the present invention contain at least
one alkaline salt, and, preferably, will contain mixtures of different
alkaline salts which are water soluble. Suitable alkaline salts or
mixtures thereof for the invention are those capable of providing the
desired pH when in aqueous solution. Most suitable are the alkaline salts
of potassium or sodium with potassium being preferred. Especially
preferred are the carbonates and bicarbonates and mixtures thereof which
are economical, safe and environmentally friendly. The carbonate salts
include potassium carbonate, potassium carbonate dihydrate, and potassium
carbonate trihydrate, sodium carbonate, sodium carbonate decahydrate,
sodium carbonate heptahydrate, sodium carbonate monohydrate, sodium
sesquicarbonate and the double salts and mixtures thereof. The bicarbonate
salts include potassium bicarbonate, sodium bicarbonate, lithium
bicarbonate and mixtures thereof.
Also suitable are the alkali metal ortho or complex phosphates. The complex
phosphates are especially effective because of their ability to chelate
water hardness and heavy metal ions. The complex phosphates include sodium
or potassium pyrophosphate, tripolyphosphate and hexametaphosphates. Also
suitable are the alkali metal borates, acetate, citrates, tartrates,
succinates, silicates and phosphonates.
Generally, the powdered flux removing compositions of the invention will
contain the alkaline salts in amounts of from about 70 to more than 99% by
weight. As set forth above, alkaline salts are utilized alone or in
combinations and in concentrations such that the resultant aqueous
solutions have a pH of from about 10, or somewhat less, to 13, preferably
from about 10 to less than 12, and, most preferably from 10.5-10.9. The
desired pH of the cleaning solution may depend on the type of flux beng
removed. Thus, the lower pH range is desirable and effective for removing
the more easily removed fluxes. However, a pH of above 11.5 is preferred
when removing the more difficult to remove solder paste fluxes.
Preferably, the powdered compositions when in solution in the wash bath
have an adequate reserve of titratable alkalinity, as least equivalent to
from about 0.2 to 4.5 percent caustic potash (potassium hydroxide), when
titrated to the colorless phenolphthalein end point, which is at about pH
8.4.
The flux removing compositions of the invention should also contain one or
more corrosion inhibitors to prevent corrosion or pitting of the
connecting tabs or solder joints, metals or other materials present on the
circuit boards being cleaned. Preferably, the corrosion inhibitor is an
alkali metal silicate salt with the sodium and potassium silicate salts
being most preferred. The alkali metal silicates which are used can be in
a variety of forms which can be encompassed generally by the formula
[Alk].sub.2 O:SiO.sub.2 wherein [Alk] represents the alkali metal and in
which the ratio of the two oxides can vary. Most useful alkali metal
silicates will have an [Alk].sub.2 O to SiO.sub.2 mole ratio of between
1:0.5 and 1:4.5. Most preferably, the [Alk].sub.2 O to SiO.sub.2 ratio is
between 1:1.6 and 1:4.0. Such silicates provide additional alkalinity to
the wash water to help cleaning. Surprisingly, it has been found that the
addition of silicate actually promotes the brightness and shininess of the
solder joints. However, other corrosion inhibitors could be used. For
sufficient corrosion protection, it is useful to add 0.1 to 10 wt. % of
the corrosion inhibitor based on the amount of powdered cleaning
composition.
At least one antifoam agent an be included in any of the flux removing
products of this invention. The antifoam agent is utilized to prevent the
formation of excessive foam caused by the rosin flux/flux removing
combination. Foam will interfere with the mechanical action of the
cleaning equipment used to wash the circuit boards. It is important, if
not critical, that the antifoam agent used herein does not act by
replacing the flux film with another residual surface film which could
affect the performance of the electronic circuit board in used. The
antifoam agent could be an agent which solely acts to inhibit foam or it
could be a surfactant which helps clean the boards and emulsify soils.
Preferred examples of antifoam agents include compounds formed by
condensing ethylene oxide with a hydrophobic base formed by the
condensation of propylene oxide with propylene glycol. The hydrophobic
portion of the molecule which exhibits water insolubility has a molecular
weight of from about 1,500 to 1,800. The addition of polyoxyethylene
radicals to this hydrophobic portion tends to increase the water
solubility of the molecule as a whole and the liquid character of the
product is retained up to the point where polyoxyethylene content is about
50 percent of the total weight of the condensation product. Examples of
such compositions are the "Pluronics" sold by BASF -- Wyandotte.
Other suitable antifoam agents that also enhance flux removal include: the
polyethylene oxide/polypropylene oxide condensates of alkyl phenols, e.g.,
the condensation products of alkyl phenols having an alkyl group
containing from about 6 to 12 carbon atoms in either a straight chain or
branched chain configuration, with ethylene oxide/propylene oxide, the
ethylene oxide being present in amounts equal to 1 to 25 moles of ethylene
oxide per mole of alkyl phenol and the propylene oxide being present in
amounts equal to 1 to 25 moles of propylene oxide per mole of alkyl
phenol. The alkyl substituent in such compounds may be derived from
polymerized propylene, diisobutylene, octene, or nonene, for example.
Also suitable are those derived from the condensation of ethylene oxide
with the product resulting from the reaction of propylene oxide and
ethylene-diamine or from the product of the reaction of a fatty acid with
sugar, starch or cellulose. For example, compounds containing from about
40 percent to about 80 percent polyoxyethylene by weight and having a
molecular weight of from about 5,000 to about 11,000 resulting from the
reaction of ethylene oxide groups with a hydrophobic base constituted of
the reaction product of ethylene diamine and excess propylene oxide, and
hydrophobic bases having a molecular weight of the order of 2,500 to 3,000
are satisfactory.
In addition, the condensation product of aliphatic alcohols having from 8
to 18 carbon atoms, in either straight chain or branched chain
configuration, with ethylene oxide and propylene oxide, e.g., a coconut
alcohol--ethylene oxide propylene oxide condensate having from 1 to 30
moles of ethylene oxide per mole of coconut alcohol, and 1 to 30 moles of
propylene oxide per mole of coconut alcohol, the coconut alcohol fraction
having from 10 to 14 carbon atoms, may also be employed.
The antifoam agents of the present invention are preferably employed in the
flux removing compositions at about 0.01 to about 10 wt. % and in the flux
removing solution in amounts of up to about 0.1 percent by weight,
preferably, about 0.01 to 0.05 percent by weight based on the total weight
of the aqueous flux removing solution. The antifoam agents thus, can be
included in the dry flux removing compositions, or added separately to the
aqueous solution so as to result in the desired concentrations during use.
The present invention also contemplates the use of one more surfactants in
the flux removing solutions in order to enhance the wetting and
emulsifying ability of the flux remover and permit maximum penetration
thereof within regions of the circuit boards most difficult to clean. The
surfactant used could be the same agent used to control the foam. Suitable
surfactants include anionic, nonionic, cationic surfactants or amphoteric
surfactants or combinations thereof. The surfactants should be soluble,
stable and, preferably, nonfoaming in use. A combination of surfactants
may be employed. The term "surfactant", as used herein, may include other
forms of dispersing agents or aids.
It has been found especially effective to use alkoxylated alcohols which
are sold under the tradename of "Polytergent SL-Series" surfactants by
Olin Corporation. Also, the polycarboxylated ethylene oxide condensates of
fatty alcohols manufactured by Olin under the tradename of "Polytergent
CS-1" have also been found effective, especially in combination with the
above Polytergent SL-Series surfactants. An effective surfactant which
also provides antifoam properties is "Polytergent SLF-18" also
manufactured by Olin. A combination of this surfactant together with the
above two surfactants has been found to provide excellent cleaning with
low foam.
Examples of other suitable surfactants are the block copolymers of ethylene
oxide and propylene oxide such as those supplied by the BASF Corporation
as Pluronics.
Ethoxylated alcohols with 8 to 20 carbons, such as those containing from 3
to 30 moles of ethylene oxide per mole of alcohol could be used as
surfactants in this invention. The monocarboxylated derivatives of these
surfactants could also be used.
Sodium or potassium salts of sulfonated benzene or naphthalene derivatives
such as alkyl benzene sulfonate, or alkyl naphthalene sulfonate or
disulfonate could be used. However, caution would have to be employed
since these surfactants might tend to impart excessive uncontrollable foam
to the wash water.
The amounts of surfactant utilized is usually small, e.g., from less than
0.01% in the wash bath, but will vary depending on the conditions and the
contamination encountered and higher surfactant levels may be employed if
so desired.
The powdered flux removing compositions of the invention may be produced by
any suitable processing technology. Most simply the dry ingredients can be
blended in a ribbon or V-shell blender while the liquid ingredients, such
as the surfactants, are sprayed on. Alternatively, the ingredients can be
agglomerated or spray dried. An advantage of the latter methods is the
production of uniform products with less tendency to segregate.
The flux removing solutions which are employed in the cleaning procedures
described herein usually contain from about 0.1 to 15, or more, percent,
preferably, from about 0.6 to 15 percent and, more preferably, from about
1 to 3 percent by weight of the powdered flux removing compositions of
this invention with the balance being essentially water. The upper limit
of concentration of the flux removing composition is not critical and is
determined by fabrication conditions, the amount of residues and the
difficulty of removing same from the circuit assemblies, etc.
The compositions of this invention are characterized by low environmental
impact, unlike the chlorinated hydrocarbon solvents and other materials
that had been used prior to this invention for printed circuit board
cleaning. For example, the alkali metal carbonate and bicarbonate salts
are naturally occurring and environmentally benign. The flux removing
compositions of the invention have biological oxygen demand (BOD) and
chemical oxygen demand (COD) values which are much lower than alternative
compositions currently available. By eliminating the need for organic
stabilizers typically utilized in aqueous concentrates for handling,
storage, and marketing, the compositions of this invention, and, in
particular, the methods of use whereby the cleaning compositions are added
directly to the wash water in the form of a powder, provide (BOD) and
(COD) levels in the aqueous effluents which are drastically reduced from
currently available compositions. As described in the Examples herein, the
flux removing compositions result in very low BODs and CODs in the rinse
water allowing the rinse water to be sewered without further treatment. In
comparison, terpenes, e.g., limonene, result in rinse water having BODs
and CODs which may require removal before sewering.
The applicability of the compositions of the invention to various aspects
of the printed circuit/wiring board fabrication process can best be
understood by a description of a representative assembly process.
The assembly manufacturing process involves the placement of components
such as integrated circuits, resistors, capacitors, diodes, etc. on the
surface of the board or their insertion through pre-drilled holes. The
components are then secured by soldering by mechanical or automatic means.
Interspersed with the soldering operations are cleaning procedures and
inspections to ensure that tape and solder flux residues than could lead
to premature circuit failure do not remain.
For the removal of rosin soldering flux deposits and other residues during
printed circuit/wiring board fabrication, the compositions of the
invention may be applied to the boards by immersion in dip tanks or by
hand or mechanical brushing. Alternatively, they may be applied by any of
the commercially available printed wiring board cleaning equipment.
Dishwasher size units may be employed, or much larger cleaning systems
such as the "Poly-Clean +" and the various "Hydro-Station" models produced
by Hollis Automation, Inc. of Nashua, New Hampshire. Other examples of
cleaning equipment which can be utilized are described in Cleaning Printed
Wiring Assemblies, Van Nostrand-Reinhold 1991, pp. 120-150.
Depending upon their design, these washers may apply the flux removing
compositions of the invention by spraying with mechanical nozzles or by
rolling contact with wetted roller surfaces. The temperature at which the
compositions may be applied can range from room, or ambient, temperature
(about 70.degree. F.) to about 180.degree. F., preferably, about
140.degree. to 170.degree. F. The flux removing compositions are diluted
with water to from as low as about 0.1 percent by weight (or volume)
concentration and up to about 15 percent by weight.
Regardless of whether the cleaning equipment used is a bath, spray nozzles,
brush or roller applicator, the cleaning composition is added directly to
the water being supplied to the equipment. Thus, the alkaline salts in
powdered form, with or without the adjuvants are added to and dispersed
directly into the supply water. Adjuvants such as corrosion inhibitors,
antifoam agents, surfactants, and the like may be coated on the powdered
alkaline salts, mixed therein if in solid form or such adjuvants may be
directly added to the wash water supply separately.
Once solder flux has been loosened and removed during a period of contact
which typically ranges from about 1 to about 5 minutes, but may be longer,
up to 10 minutes, the boards are taken from the flux removing solution.
Another advantage of the instant invention is that the flux removing
solutions need not be flushed with solvents as with the processes of the
prior art. Herein, the boards may simply be flushed with water for a
period of up to about 2 minutes. Deionized water is preferred. The optimal
rinsing time varies according to the kinds of surfactants and the
concentrations of the flux removing solutions used and can easily be
determined by routine experimentation.
The cleaned boards are then dried, preferably with forced air. Drying is
expedited if the air is warmed, preferably to above about 100.degree. F.
The efficacy of rosin soldering flux removal from printed wiring boards is
such that the boards meet stringent military specifications for low
resistivity after cleaning. For example, the boards meet the Mil-P-28809A
standard for low resistivity of the solvent extracts resulting when the
contamination has been removed from a circuit board cleaned according to
Mil-P-55110C. The resistivity of such solvent extracts after the cleaning
of the boards is complete is most easily determined with an Omega Meter.
Omega Meter is the registered trademark of Kenco Industries, Inc.,
Atlanta, Ga., for a microprocessor-controlled contamination test system
that rapidly measures changes in resistivity due to contaminating ions.
The results of Omega Meter measurements are expressed in equivalent units
of ug NaCl/in.sup.2 or its metric equivalent. According to MIL-P-28809A,
the acceptable resistivity value for a cleaned board is equivalent to 2.2.
ug NaCl/cm.sup.2 or 14 ug NaCl/in.sup.2, but far better results are
routinely obtained after solder flux has been removed with the flux
removing solutions of the present invention. A value of about 0.31 ug
NaCl/cm.sup.2, or 2.0 ug NaCl/in.sup.2, or even less, is typical.
The flux removing solutions of this invention are also effective in
removing other undesirable and deleterious substances and residues. One
particularly troublesome substance is the residue left by adhesive tape
used during fabrication of the electronic circuit assemblies.
During the process of gold plating connecting tabs to improve corrosion
resistance, tin-lead residues must first be removed from the unplated
tabs. Removal of these residues is carried out through the use of etching
chemicals that can damage other unprotected printed circuit/wiring board
components. To protect vulnerable components from the etching chemicals,
boards are wrapped on both sides with an adhesive plating tape which forms
a shield or splash guard for all but the exposed tab area. The etching
chemicals then remove the tin-lead residues on the tabs, a nickel plate is
applied as a base for the gold, and gold plating of the tabs is finally
carried out. The adhesive plating tape which is maintained in place
through all of these etching and plating steps, is then removed. When the
tape is removed following the nickel and gold plate step, it is at this
point that the cleaning compositions of the invention may most
advantageously be used.
Thus, following removal of the tape, a silicone-based and/or rubber-based
adhesive residue may remain on the board. This residue may easily be
removed by employing the compositions of the invention under the same
conditions described above for solder flux removal. The exact operational
parameters will be determined by the nature of the adhesive residue and
the tenacity with which it adheres to the board, but the conditions
described above are generally effective. As in the case of solder flux
removal, treatment of the board with the flux removing solutions of the
invention is generally followed by water flushing and air drying.
The efficiency of removal of adhesive residues from printed circuit/wiring
boards by the compositions of the invention is such that no residues are
visible after cleaning. A simple 5-10.times. stereomicroscope can
facilitate visual inspection for tape residues following cleaning.
EXAMPLES 1-4
The following examples illustrate useful powdered compositions which can be
utilized in any of the known ECA cleaning methods and apparatus. The
individual ingredients of the useful compositions are set forth in Table I
below. All compositions are formulated by spraying liquid ingredients onto
the solid carbonate and bicarbonate or phosphate powders or by spray
drying the ingredients to uniform powders. Table I also includes the
suggested usage concentrations of each composition in the wash bath.
TABLE I
______________________________________
Examples
1 2 3 4
______________________________________
Potassium Carbonate
70.6 36.0 0.0 30.0
Sodium carbonate
12.0 50.0 31.0 55.0
Sodium bicarbonate
12.0 5.0 0.0 0.0
Sodium tripolyphosphate
0.0 0.0 35.0 0.0
Trisodium phosphate
0.0 0.0 15.0 0.0
Sodium silicate (2.4
2.0 6.0 15.0 6.0
SiO2:Na.sub.2 O)
Sodium metasilicate
0.0 0.0 0.0 4.0
Polytergent CS-1
0.8 0.0 1.0 1.0
Polytergent SL-62
2.0 0.0 1.0 2.0
Polytergent SLF-18
0.6 0.0 1.0 2.0
Pluronic 25R2 0.0 0.5 0.5 0.0
Plurafac RA30 0.0 0.5 0.5 0.0
Total 100.0 100.0 100.0 100.0
Suggested 1-3% 2-6% 0.6-2% 1-3%
Usage Concentration
______________________________________
Polytergent is a tradename of Olin Corporation.
CS1 is a carboxylated, ethoxylated fatty alcohol mixture,
SL62 and SLF18 are alkoxylated fatty alcohol mixtures.
Pluronic 25R2 is a tradename of BASF Wyandotte and consists of a block
copolymer of ethylene oxide and propylene oxide.
Plurafac RA30 is a tradename of BASF Wyandotte and consists of an
alkoxylated surfactant alcohol.
EXAMPLE 5
The powdered composition of Example 1 is utilized to clean test wiring
boards using a "Polyclean +" machine manufactured by Hollis Automation,
Inc., of Nashua, New Hampshire.
The cleaning sequence comprises the operations of loading, washing, drying,
first rinsing, final rinsing and high speed drying carried out in
succession. The washing operation utilizing cleaning solutions of the
invention is done in two stages, i.e., a first regular wash with a spray
nozzle manifold which directs a regular wash spray at 40 psig followed by
a "hurricane" spray at 80 psig. The cleaning solutions are maintained at
160.degree. F. The rinses are also two stage operations; the first at 40
psig regular rinse followed by an 80 psig "hurricane" rinse with the rinse
water having a temperature of 160.degree. F. A final rinse is effected
under substantially the same conditions. The circuit boards are subjected
to Alpha air knife drying after the washing and final rinse stages. In air
knife drying, turbine propelled air shears fluids from the boards'
surfaces.
Cleaned and dried boards are evaluated for cleaning efficiency visually.
The visual test method uses a dyed flux and carrier base injected between
glass components and a glass board. This provides excellent access for
visual inspection. The analysis is further quantified by placing the board
and components against a grid. Each block of the grid is then read as
being completely clean or containing residue.
The test method utilizes straight flux and carrier from a rosin mildly
activated (RMA) flux or paste. It is essentially the solder paste minus
the solder. "Carrier" refers to both the flux paste and all other
additives included in solder paste, except the solder. This carrier is
then injected with red dye so that visual examination can be made more
rapidly. The dye does not affect the carrier density or melting
properties. The dyed carrier is then injected under the glass components
on specially made test boards. RMA solder paste is not considered an
aqueous-compatible flux. The test boards are constructed of glass. A
1".times.1" square coupon that simulates the component is mounted onto a
glass substrate. The coupon is glued in place by first laying shim stock
of the desired standoff height on the glass. Next, the glue is applied and
the coupon set in place until it dries. When dry, the shim stock is
removed. Six coupons are mounted on a single board at 1/2" spacing. The
interior coupons are further shielded from any nozzles by the first
coupons in the placement array.
The flux carrier stock is injected under each coupon to entirely fill the
inch-square area. Flux is also added to the area surrounding each coupon.
The board is IR-reflowed at a typical dwell time of five minutes at reflow
temperature. All boards are then stored for 24 hours at ambient
temperature prior to cleaning. Reflowing and storing increases cleaning
difficulty by allowing the board to cool and the flux carrier to set up.
Prior to reflow, the entire area under the coupon is filled with the dyed
flux carrier. During reflow, a small percentage of the area under the
coupon develops voids due to expansion and escape of flux volatiles. The
area under the coupons filled with baked-on residue is measured prior to
cleaning. The application method causes most of the flux to be bridged
across the component standoff height. These regions entirely filled with
flux are the most difficult to clean. They are also much less likely to
occur in actual manufacturing processes since much less flux is applied.
For the purposes of this test, however, no special measurement
qualification is given to this category. By regarding all areas with flux
trapped under them as the same, the test method is made more rigorous.
This method is directed toward the measurement of cleaning effectiveness,
which is defined as the percentage of residue removed. This aqueous
cleaning test method is described more fully in a publication by Janet R.
Sterritt, "Aqueous Cleaning Power," Printed Circuit Assembly, Sept. 1989,
pp. 26-29.
The Polyclean+ machine is operated so that all wash water for the two-stage
nozzle spray during cleaning comes from a supply which includes the
composition of Example 1. The composition is added as a powder directly to
the water supply so as to provide a concentration of 1.5 weight percent of
the composition in the wash water. The sequence of the machine is then
initiated. The wash cycle time is 1.5 minutes with a wash water
temperature of 160.degree. F.
Visual evaluation of the board by grid analysis shows that over 97 percent
of the flux is removed.
EXAMPLE 6
The following example illustrates the improved environmental impact the
powdered cleaning compositions of the present invention provide relative
to present commercial cleaning formulations. Thus, the composition of
Example 2 above is added to a wash solution at a concentration of 2 wt. %.
At this concentration, it is calculated that only about 400 ppm of COD is
added to the wash water. In comparison, a typical commercial aqueous flux
removing composition adds 100,000-500,000 ppm of COD to the wash water. It
can be seen that by eliminating the need for organic stabilizers, the COD
of the wash water and consequently wash effluents can be drastically
reduced utilizing the powdered compositions of this invention.
EXAMPLE 7
Similar to Example 6, the composition as set forth in Example 4 above is
added to a wash solution at a concentration of 3 wt. %. Example 4 has a
higher concentration of surfactants than the other examples set forth in
Table 1. At the concentration designated, Example 4 is calculated to add
3,500 ppm of COD to the wash water. As can be seen, this compares most
favorably to the presently available commercial flux removing compositions
.
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